Variable displacement compressors are being established as the preferred means for compressing refrigerant in air conditioning systems of present day motor vehicles. The compressor typically includes a plurality of cylinders, each adapted to receive a piston for reciprocal compressing action. As is commonly known, the piston includes a piston head sealed around the inner annular surface of the cylinder, and a piston rod driving the piston head to provide the desired compressive action.
A typical design places a number of radially disposed cylinders equally angularly spaced about the axis of a central drive shaft. A swash plate or wobble plate is received about and is operatively connected to the drive shaft. The swash plate responds to the drive movement by nutating about the drive shaft, and this motion then converts to a linear reciprocating motion of the individual pistons. This motion and cooperative interaction dictates that the simplest head configuration is of the type defining a curved, annular sealing surface, and the most effective connection between the swash plate and the end of the associated piston rod is a ball joint.
Historically, pistons for swash plate compressors of this type have been of unitary construction, machined from a durable metal body, such as a steel alloy. This prior art construction provides the desired strength, but unfortunately has an undesirably high machining cost, and a relatively high mass. It can be appreciated that substantial momentum and inertia forces are produced, due to the extreme acceleration imparted to a compressor piston. Accordingly, there is a need to reduce the mass of the piston itself in order to reduce the inertia forces generated by the piston action.
Recent designs have concentrated efforts away from such unitary piston fabrication, and more to making composite piston assemblies. A common approach is to attach an aluminum piston head to a steel connecting rod. In this way, the overall piston mass is reduced by substituting a lighter weight aluminum for the piston head.
A typical design configuration for this type of composite piston assembly is provided by a cast piston head having a central spherical pocket. In turn, the connecting rod is fabricated with an integral ball fixed to the end for receipt in the pocket of the piston head. In this way, a swivel joint engagement between the connecting rod ball and spherical pocket of the piston head accommodates the radial and axial displacements resulting from the nutating motion of the swash plate. For purposes of the present disclosure, this piston/connecting rod configuration will be referred to as the "ball-type".
While this "ball-type" design effectively reduces piston mass, further improvements are needed. For example, it is desired to have a simplified and reliable piston design, and one that greatly reduces the fabrication time. A move back to the "fixed attachment" design, that is without a ball joint in the piston, but wherein the head is curved to accommodate the swivel/reciprocating movement of the assembly is indicated. In doing so, the connecting rod does not have to have either a machined spherical pocket, or a journal washer and retainer clip to close the pocket. These two extra parts of the piston head assembly that substantially increase the cost, and complicate the manufacturing process, are not needed.
In the fabrication process of the ball-type connection, the inner surface of the spherical pocket must not only be machined, but finished to remove burrs or any other surface defects produced in the casting process. This need to provide a smooth, friction-reduced surface for wear-compatible engagement with the steel ball of the connecting rod is expensive. Thus, it should be appreciated that this fabrication process for another reason requires an excessive amount of time, thereby also increasing the manufacturing cost. The cost is further increased by having to provide lubrication ports to maintain the low friction working joint.
Whenever individual parts, such as the journal and retainer clip of the ball-type joint, are designed for secure attachment, rigid dimensional tolerances must be achieved in order to pass acceptable quality control standards. Thus, parts that do not meet the close tolerance requirements must be discarded as scrap, thereby driving up the manufacturing cost more.
One successful example of a composite piston design approach with a "fixed attachment" head/connecting rod is disclosed in U.S. Pat. No. 5,022,313 issued Jun. 11, 1991, and entitled Composite Piston Assembly for Automotive Air Conditioning Compressor (assigned to the assignee of the present invention). The piston assembly starts with a separate piston head and connecting rod. The connecting rod is fabricated from aluminum and the piston head is a reinforced plastic, molded to the end of the connecting rod. Accordingly, the lightweight construction reduces the inertia forces on the piston head, and allows higher compressor speeds to be attained.
While this molded-type, fixed attachment piston design effectively enhances compressor performance by accommodating higher compressor speeds, like the ball-type design, further improvements are needed. For example, a shorter fabrication time and cost are desired. In order for the molded plastic to effectively grip and adhere to the connecting rod, the rod must first be prepared by providing a knurled surface for the plastic to bond. It is observed that the time required to prepare the rod surface, as well as the time needed to insert the rod in the mold, inject the plastic, and wait for the plastic to set up and harden, is unnecessarily lengthy.
It is also desired to provide a piston assembly having an improved reliability. As indicated, in the ball-type of piston assembly, the steel ball of the connecting rod swivels and rotates within the spherical piston pocket during operation. Over the life of the compressor, this combined swivel/rotary motion causes wear, especially on the journal and the retainer clip of the joint, which can eventually develop into a loose connection, and even ultimately fail. Accordingly, there is clearly a need for a new lightweight, composite piston assembly of the type having a fixed attachment and a related, more efficient fabrication process, whereby a more cost effective and reliable piston design is realized.